RFR: Make object pinning safepoint aware

[Zhengyu Gu]

Thread running native code does not usually participate safepoints, but 
the write barrier in object pinning code can potential interfere with 
safepoints and mutate the heap, so we need to make it safepoint aware.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/obj_pin_safepoint/webrev.00/index.html

Test:
   hotspot_gc_shenandoah fastdebug + release on x64 and aarch64.
   + new stress test.

Thanks,

-Zhengyu